Surface-carbonitrided stainless steel part excellent in wear resistance and its manufacturing method

ABSTRACT

A steel part according to the present invention is a part in which a surface of an austenitic stainless steel containing 3 to 20 mass % of Mn was carbonitrided to be hardened. By setting Vickers hardness of the surface to 1350 HV or more and setting a depth of a hardened layer having 1000 HV or more from the surface of the steel to 10 μm or more, when the part according to the present invention is applied to a part required for sliding and wear resistance particularly, the service life can be improved significantly. Further, since the manufacturing method is performed by only heating in a gas atmosphere, a large number of parts can be simultaneously treated. Thus the stainless steel parts of the present invention can be adopted to wide fields as stainless steel parts required for wear resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface hardened austenitic stainlesssteel part for improving wear resistance and its manufacturing method.

2. Background Art

Austenitic stainless steels have been utilized in various fields,required for corrosion resistance such as food machines, machines forchemical fields, plants and auto-motive engines and others. Among themthere are many applications, also required for wear resistance, likesliding machine parts such as shafts and the like, valves and the like,gears and the like and so on. Tb improve this wear resistance, hardeningby heat treatment such as quenching or the like, or surface-hardeningsuch as carburizing, carbonitriding or the like is often applied tomachine structural carbon steels, alloy steels and tool steels and thelike.

In a case of austenitic stainless steel it is not hardened by quenching.Thus since sufficient hardness cannot be obtained by carburizing in theaustenitic stainless steel, when it is necessary to harden the surfaceof the stainless steel particularly, wet plating such as hard chromiumplating or the like, coating of a hard layer by PVD (Physical VaporDeposition) or nitriding of the surface or the like is performed.However, coating of a film by plating or PVD or the like has a drawbackin adhesive properties of the film with the base material. Particularly,when a surface pressure is increased, the coating cannot be appliedstably.

On the other hand, a surface nitriding treatment is a method in whichnitrogen is penetrated through a surface to harden it. Stainless steelcontaining much Cr (Chromium) is generally difficult to nitride for thesake of the presence of an oxide film. However, the nitriding of thestainless steel becomes easy by use of a hydrochloric acid treatmentprocess, a halide treatment process, or ion-nitriding treatment processor the like. These processes are often utilized as a surface-hardeningmethod of austenitic stainless steel.

The surface hardening of the austenitic stainless steel is needed forimproving its fatigue strength. However, it is further needed for thesake of improving its wear resistance. The improvement of the wearresistance suppresses wear on a sliding surface of a sliding part toimprove durability thereof. Additionally the improvement of the wearresistance has such effects that wear loss in a tool for polishing orcutting is reduced and scratching of a surface of a stainless steel partis suppressed.

Nitriding treatment is not necessary to quench unlike the carburizingtreatment. Accordingly, in the nitriding treatment the surface can behardened by a comparatively low temperature treatment. However, there isthe most suitable temperature for increasing the surface hardness, andif the thickness of a hardened layer is increased at the temperature, ittakes long time for nitriding. Otherwise, if the temperature isincreased the thickness of the hardened layer can be increased. However,the obtained surface hardness is decreased.

Further, to improve the wear resistance of sliding parts, higher surfacehardness of the parts is better. However, when a surface hardeningtreatment is performed at a decreased temperature, a hard layer such asa compound layer can be produced on the surface layer of the part.Nevertheless, the hard layer is brittle and the wear resistance is notnecessarily improved.

As described above, although the method using the nitriding is importantas a surface-hardening method of the austenitic stainless steel,sufficiently satisfactory hardening method cannot be necessarilyobtained.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an austeniticstainless steel part excellent in wear resistance, which is used in asliding portion or the like, has high surface hardness and hassufficient hardness in a portion just below the surface, and itsmanufacturing method.

The present inventors have studied variously to improve the performanceof austenitic stainless steel surface-hardened by gas nitriding.Although the surface nitriding process of the stainless steel includesion nitriding, the nitriding is performed under a reduced pressure andthe treatment speed is slow and even the shape to be nitrided can belimited. On the contrary, gas nitriding can be treated in high amounts,and it has been considered that the gas nitriding is suitable for massproduction.

Therefore, regarding the surface hardening of the austenitic stainlesssteel by gas nitriding, steels having various different compositionswere prepared, and influences of the gas compositions and treatmentconditions on the surface hardness and hardened depth of the steel bygas nitriding have been studied variously.

In the case, while using a furnace that can shut out the outside air,gas containing fluoride of 10 volume % of NF₃ and the remaining of N₂was led to the furnace and the steels were maintained for thirty minutesso that the surfaces of the steels were activated. After that nitridinggas containing NF₃ was injected into the furnace and temperature andtime were variously changed for nitriding treatment.

The surface hardness of the steel by nitriding generally reaches about1200 to 1300 HV. However, although a harder compound layer can be formedon the surface of the steel by treatment conditions, it is generallyconsidered that the compound layer is so brittle that it cannot beutilized. In examinations using these austenitic stainless steels, steelhaving a surface hardness of 1350 HV or more at Vickers hardness wasfound.

This hard surface layer is not a brittle compound layer, which has beenobtained in machine structural steels, ferritic stainless steels and thelike, but it has sufficient toughness. Thus, after preparing test piecesfor wear tests, wear resistances were checked. As a result, it has beenrecognized that the wear resistances are extremely excellent.

Then, the thus hard surface obtained steel members were examined indetail. As a result, the following points have been found.

-   -   (a) A compound layer is recognized on the surface layer.    -   (b) The steel is an austenitic stainless steel containing much        Mn (Manganese).    -   (c) The atmosphere used in nitriding treatment contains, in        addition to NH₃, carburizing gas such as RX gas or the like.

In the case of a machine structural steel or a ferritic stainless steel,a compound layer often appears on the surface layer depending ontreatment conditions. It is considered that this compound layer wasformed by increasing the content of active nitrogen on the surfacelayer, produced by decomposition of NH₃ due to the advancing ofnitriding, and by nitriding Fe, Cr or the like with the increasednitrogen.

However, in the case of an austenitic stainless steel such a compoundlayer does not usually appear. It is considered that the solubility ofnitrogen in an austenitic phase of the steel is significantly largerthan in a ferritic phase of the steel, and nitrogen is diffused into thesteel and an increase of nitrogen on the surface of the steel isdifficult to arise whereby a compound layer is not formed.

However, in the case of an austenitic stainless steel containing muchMn, when it is treated in a nitriding and carburizing atmosphere, acompound layer appears on the steel. The treatment-performed test piecesexhibited excellent wear resistance.

The reason of such a result is not necessarily clear. However, if thefacts of the above-mentioned (a), (b) and (c) are also considered, thefollowing assumption is obtained.

First, hardening by nitriding treatment which is performed at a lowertemperature compared with usual carburizing temperature is obtained bythe formation of fine nitride precipitates and an increase of solublenitrogen. In the cases of the machine structural steel and the ferriticstainless steel, since the steels have ferritic phases and smallsolubility of nitrogen, a compound layer is liable to be formed.However, since in a portion just below the compound layer the content ofnitrogen is not increased, the hardness of the steel is low and there isa large difference of hardness between the compound layer and theportion just below the compound layer.

Therefore, the steel cannot hold a hard and brittle compound layersufficiently and the hard compound layer is easily broken by smallstress. Thus, only brittleness is prominent and the hard compound layeris not made full use of.

On the contrary, in the case of the austenitic stainless steel anaustenitic phase has significantly larger solubility of nitrogen thanthe ferritic phase. It was seemed that the appearance of a compoundlayer in the austenitic steel containing much Mn was due to the factthat Ni is small.

The purpose of causing much Mn to contain into the austenitic stainlesssteel resizes in suppressing the use of expensive Ni, and when much Mnis contained in the steel, the Ni content is surely low. Ni is said tobe an element, which generally prevents nitriding, and since Ni issmall, penetration of nitrogen and penetration of carbon become easy.Therefore, it is assumed that the nitrogen content near the surface ofthe steel during nitriding is further increased as compared with a casewhere the austenitic stainless steel contains a small amount of Mn and alarge amount of Ni.

Further, when carburizing gas containing CO and CH₄ such as an RX gasexists in a nitriding atmosphere, carburizing advances simultaneously.Thus, nitrogen dissolved in the steel has the same effect as a casewhere the nitrogen content was increased by the presence of dissolvedcarbon and a compound is easy to be formed on the surface of the steel.A portion just below a compound layer having an austenitic phasecontains dissolved nitrogen and dissolved carbon more than the case of aferritic phase, since the solubility of the portion just below thecompound layer is large.

Thus, the solubility content of nitrogen is high since the austeniticsteel has an austenitic phase, and the penetration of nitrogen isactively performed since Ni content is low. Additionally, a compoundlayer is easy to be formed by the penetration of carbon. Further, thehardness of the portion just below the compound is significantlyincreased since large amounts of fine carbide and nitride having highsolid solubility content are formed.

Therefore, it is considered that since the compound layer is held by thelower layer having sufficient strength, the brittleness of the compoundlayer is compensated, resulting in that the compound layer has become asurface-strengthened layer excellent in wear resistance. In order toimprove wear resistance, it is important to have higher surface hardnessas well as a suitably-thick strengthened layer having intermediatehardness between the base and the hard surface layer.

Therefore, various investigations of the compositions of steels,nitriding treatment conditions, various properties of the obtainedsurface hardened parts and the like were performed with respect to anaustenitic stainless steel containing much Mn. Based on the result of areview limits by which such results can be obtained were clarified andthe present invention was accomplished. The gist of the presentinvention is as follows.

(1) A surface-carbonitrided stainless steel part excellent in wearresistance, in which Mn of 3 to 20 mass % is contained in the steel anda surface of the steel was carbonitrided to be hardened, wherein Vickershardnesses of the surface are 1350 HV or more and depths of a hardenedlayer having 1000 HV or more are 10 μm or more.

(2) A manufacturing method of a surface-carbonitrided stainless steelpart excellent in wear resistance according to the above mentioned (1),wherein after a stainless steel part containing 3 to 20 mass % of Mn,molded in a required shape, was surface activated in an atmospherecontaining halogen gas or halide gas, the obtained stainless steel partis carbonitrided at 430 to 600° C. in an atmosphere containing NH₃ andcarburizing gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A steel, which is manufactured to a part of the present invention, is anaustenitic stainless steel containing 3 to 20 mass % of Mn. The reasonfor Mn content of 3 mass % or more is that in a case of an austeniticsteel when Mn is decreased, Ni content is increased and surfacehardening hardness due to nitriding is not significantly increased. Onthe other hand, when the Mn content is increased the corrosionresistance for the austenitic stainless steel becomes deteriorated.Thus, the Mn content is set to at most 20 mass %.

Such stainless steels include for example SUS 201, SUS 202, SUS 304J2,SUH 35, SUH 36 and the like in the JIS standard. The compositions otherthan Mn are not particularly limited if they are in ranges, which belongto the austenitic stainless steels. Nevertheless, it is preferable thatthe Ni content is, if possible, smaller than the Mn content since thereis a possibility that sufficient surface hardness of the steel cannot beobtained.

The hardness of a carbonitrized surface is set to 1350 HV or more. Thisis because sufficiently high wear resistance cannot be obtained when thehardness thereof is lower than 1350 HV. Thus, in addition to the surfacehardness of 1350 HV or more, the depth of a hardened layer whosehardness is 1000 HV or more is set to 10 μm or more.

This is because in a case where the hardness of a hardened layer justbelow a surface compound layer is lower than 1000 HV, when the depth ofthe hardened layer is less than 10 μm, any surface hardness not onlyreaches a level lower than 1350 HV, but also a surface compound layerbecomes brittle and wear resistance deteriorates.

The austenitic stainless steel part surface hardened in theabove-mentioned conditions cannot be obtained by only nitriding. Thus itis necessary to manufacture the part by carbonitriding. As the surfacenitriding of the austenitic stainless steel there is a method in whichthe surface of the steel is heated in an atmosphere containing halogengas or halide gas to activate the surface, and nitriding gas containingNH₃ is introduced into a furnace and the heated surface of the steel isnitrided. In the present invention carbonitriding is performed accordingto the method of using the halogen or halide.

First, by using a heating container which may be hermetically closed, anaustenitic steel is heated at 200 to 550° C. for 10 minutes to 3 hoursin atmosphere containing 0.5 to 20 volume % of halogen gas or halide gassuch as F₂, Cl₂, HCl or NF₃ and the balance of nitrogen, hydrogen orinactive gas to activate the surface of the steel.

After the surface of the steel was activated, the surface iscarbonitrided by heating at a temperature range of 430 to 600° C. for 20minutes or more in a mixed gas atmosphere containing NH₃ for nitridingand CO or CH₄ for carbonizing.

These carbonitriding atmospheric gases include 10 to 95 volume % of NH₃and 5 to 30 volume % of one of CO or CH₄, or both of them. The reasonfor 10 volume % or more of NH₃ is that if NH₃ is less than 10 volume %,nitriding cannot be performed sufficiently and a hardened layer cannotbe obtained. 100 volume % NH₃ may be used for the purpose of nitriding.However, since it is necessary to use a carbonizing gas, NH₃ iscontained by at most 95 volume %.

For the purpose of carbonizing, 5 volume % or more of one of CO or CH₄or both of them is needed. However, when these gas ratio is tooincreased, soot is generated. Accordingly, the gas content is set to atmost 30 volume %.

The carbonitriding atmospheric gas may contain NH₃ and CO or CH₄ byenough contents for nitriding and for carbonizing the surface of thesteel, and other components may be inactive gas, hydrogen, nitrogen orother hydrocarbon gases and the like, which are not limited. Further, ifthe above-mentioned composition ranges are satisfied as in a case whereNH₃ is mixed to RX gas, carbonitriding gas may be prepared by mixingcarburizing gas, which has been used, with NH₃.

When the carbonitriding temperature is less than 430° C. or less,surface hardness of 1350 HV or more cannot be obtained. Additionally, adevelopment of a hardened layer of 1000 HV or more becomes insufficient.This is because nitriding advances but carbonizing does not advancesignificantly. When the carbonitriding temperature reaches 430° C. ormore, these surface hardness and hardened layer can be obtained.However, when the carbonitriding temperature exceeds 600° C., not only asurface hardness of 1350 HV or more cannot be obtained, but alsocorrosion resistance as a stainless steel is reduced.

Further, when the carbonitriding time is less than 20 minutes, there isa possibility that a surface compound layer cannot be obtained, and asurface hardness of 1350 HV or more cannot be obtained. If thecarbonitriding time is 20 minutes or more, the limitation of the time isnot needed. Then the carbonitriding time is increased, the thickness ina hardened layer of 1000 HV or more can be increased. However, wearresistance is not improved more than a certain level and it ispreferable that the carbonitriding time is within at most 50 hours sincecorrosion resistance can be deteriorated.

Examples of stainless steel parts, which are required for wearresistance and to which the present invention is effectively applied,are described as follows. Those are sliding mechanical parts includingan engine valve, a compressor shaft, a compressor vane, a piston ring, abearing ball, a micro motor shaft, a motor shaft, and the like. Further,fluid wear resistant parts include a filter mesh, a nozzle, a valve, apiping joint, a reducer, a pump and the like. Furthermore, fasteningparts include a bolt, a nut, a screw, a tapping screw, and the like.Further, tools and the like include a dresser, a cutting saw, a wiresaw, a saw, a drill and the like. The stainless parts can also beapplied to an extrusion mold, a die cast mold, an injection die and thelike.

EXAMPLES

Stainless steels having compositions shown in Table 1 were used. First,they were cut to prepare disk-shaped test pieces of 35 mm in diameterand 10 mm in thickness. In a case where the test piece is used as arotating test piece for an Amsler wear test, the circumferential surfaceof the disk is further polished in a mirror surface to remove edges. Theobtained test pieces were heated at 300° C. and the heating wasmaintained in an atmosphere containing NF₃ to perform nitriding orcarbonitriding thereby hardening the surfaces of the test pieces. Theatmospheric gases, temperatures and treatment time duringsurface-nitriding are shown in Table 2.

In reference to surface hardened test pieces the surface hardness wasmeasured at Vickers hardness (HV0.1) of test force of 0.9806 N, and thehardness distribution in cross-section was measured at Vickers hardness(HV0.05) of test force of 0.4903 N. The concave mark obtained after themeasurement of surface hardness, was observed with an optical microscopeat 100 fold and when generation of chipping or a crack was recognized,the test piece was determined to be brittleness failure.

Amsler wear test was performed as follows. In two cylindrical rollingabrasion testers circumferential surfaces of the above-mentioned testpieces were pressed against cylindrical metal surfaces (made of SKH52)of 35 mm and 50 mm in diameter at 150 kg, and the abrasion testers wererolled in the same direction in sliding portions. Then the sliding speedwas set to 0.12 m/sec and specific abrasion wear [mg/(m·sec)] wasobtained without lubrication.

TABLE 1 Chemical Composition (Mass %) Steel [Balance: Impurities and Fe]Mark C Si Mn Cr Ni Mo N Remarks A 0.05 0.2 9.5 13.5 2.5 1.0 0.15 Rangeof the present invention B 0.05 0.5 10.0 11.0 5.0 1.0 0.05 Range of thepresent invention C 0.05 0.5 10.0 22.0 5.0 2.0 0.10 Range of the presentinvention D 0.03 0.5 *2.5 25.0 7.0 3.5 0.30 Out of the range Mark *shows out of range defined in the present invention.

The results of the abrasion test are shown in Table 2 on the next page.The test pieces of test Nos. 1 to 3 in which an austenitic stainlesssteel of a sufficiently high Mn content was used and carbonitriding wasperformed as a surface hardening treatment, exhibit excellent lowspecific abrasion wear. This is assumed to be the facts that the steelhas a high surface hardness, which exceeds 1400 HV, and a hardened layerdepth having 1000 HV or more.

TABLE 2 Hardened Depth Surface Specific Surface Hardened TreatmentSurface of 1000 HV Hardened Abrasion Test Steel Temperature TimeHardness or more Layer Wear No. Mark Atmosphere (° C.) (h) (HV0.05) (μm)Brittleness [mg/(m · s)] Remarks 1 A 50% NH₃ + 570 3.0 1500 50 Good 0.1Example of the 50% RX present invention 2 B 50% NH₃ + 540 2.0 1450 18Good 0.1 Example of the 50% RX present invention 3 C 50% NH₃ + 570 3.01500 50 Good 0.1 Example of the 50% RX present invention 4 *D  50% NH₃ +570 3.0 *1200 50 Good 0.5 Comparative example 50% RX 5 A 50% NH₃ + *41048.0 *1330 7 Bad 0.5 Comparative example 50% RX 6 A *100% NH₃ 570 3.0*1300 50 Bad 0.6 Comparative example Mark * shows out of range definedin the present invention

On the contrary, even if a test piece of test No. 6 is the austeniticstainless steel of the high Mn content, the surface hardening treatmentis not carbonitriding. Accordingly, the surface hardness of the testpiece of test No. 6 is not sufficient and is brittle. Next, in a testpiece of test No. 5 since a carbonitriding temperature is low a hardeneddepth of 1000 HV or more is 7 μm, which is shallow. Further, although asurface hardening treatment of carbonitriding in a test piece of testNo. 4 was performed, since the test piece of test No. 4 is an austeniticstainless steel of a low Mn content, the surface hardness is less than300 HV. All cases of test Nos. 4 to 6 exhibit large specific abrasionwear and worse results as compared with the cases of test Nos. 1 to 3according to the present invention.

As described above, by applying the parts of the present invention tovarious mechanical parts, to which austenitic stainless steel isapplied, particularly to parts required for sliding and wear resistance,their service life can be improved significantly.

INDUSTRIAL APPLICABILITY

According to the surface carbonitrided stainless steel part of thepresent invention and its manufacturing method, by setting a Vickershardness of the surface to 1350 HV or more and setting a depth of ahardened layer having 1000 HV or more from the surface of the steel to10 μm or more, when the part according to the present invention isapplied to a part required for sliding and wear resistance particularly,the service life can be improved significantly. Further, since themanufacturing method is performed by only heating in a gas atmosphere, alarge number of parts can be simultaneously treated. Thus the stainlesssteel parts of the present invention can be adopted to slidingmechanical parts, fluid wear resistant parts, fastening parts and toolsand the like as stainless steel parts required for wear resistance.Therefore, the present invention can be applied to wide fields.

1. A surface-carbonitrided stainless steel part excellent in wearresistance, in which 3 to 20 mass % of Mn is contained in the steel anda surface of the steel was carbonitrided to be hardened, wherein aVickers hardness of the surface is 1350 HV or more and a compoundprimarily comprised of a nitrogen compound containing carbon is formedon said surface and yet a depth of a hardened layer having 1000 HV ormore from the surface of said steel is 10 μm or more.
 2. A manufacturingmethod of a surface-carbonitrided stainless steel part excellent in wearresistance according to claim 1, wherein after a stainless steel partcontaining 3 to 20 mass % of Mn, molded in a required shape, was surfaceactivated in an atmosphere containing halogen gas or halide gas, theobtained stainless steel part is carbonitrided at 430 to 600° C. in anatmosphere containing NH₃ and carburizing gas.